The present invention relates to a process for producing a heat-resistant Ni3Al intermetallic compound having an excellent room-temperature ductility. More specifically, it relates to a process for producing a thin Ni3Al foil which has a thickness of 200 microns or less and which is excellent in high-temperature strength, oxidation and corrosion resistances, and room-temperature ductility.
With respect to the improvement of the brittleness of Ni3Al as a heat-resistant structural material, a method of adding trace boron which was studied in Tohoku University in 1979 is known. In the past, there was an example of forming a plate having a thickness of approximately 500 microns. However, an actual problem that brittleness tends to occur at a high temperature even with the addition of boron still remained unresolved. Further, Ni3Al containing trace boron was not suited for production of a foil because it had a low room-temperature tensile elongation of approximately 30% and a high yield stress and is consequently low in rolling moldability.
Afterwards, Oak Ridge National Laboratory also studied the improvement of the alloy composition for reducing this material to practical use.
However, in any of the approaches in the past, the problem was to use the material in the form of a plate, a block or a wire, and no attempt was made to use it for production of a foil.
Unlike ordinary metallic materials, the strength of Ni3Al increases with increasing temperature. At a temperature of 800xc2x0 C. (or 1,073 K), the strength reaches five times as high as that at room temperature as shown in FIG. 2. For this reason, the material has been considered to be used as a heat-resistant material, in turbine blades, boiler pipes, fuel cladding pipes for a reactor and aerospace materials.
However, conventionally melt and cast Ni3Al are so brittle as to be broken soon after yielding. Such brittleness in the vicinity of room temperature which is common to intermetallic compounds.
Accordingly, it is difficult to produce a foil having a thickness of 200 microns or less by cold rolling. It has been so far considered quite difficult, and has not yet been realized.
Under these circumstances, the invention has been made. The objective of the invention is to provide a process in which a thin Ni3Al foil having a thickness of 200 microns or less can be produced by cold rolling from an Ni3Al intermetallic compound excellent in high-temperature strength, oxidation and corrosion resistances, and high ductility.
Another objective of the invention is that the foil is applicable to the production of lightweight, heat-resistant structural materials such as a honeycomb structure and a laminated complex material and many other functional materials.
In order to accomplish the foregoing and other objectives, the invention first provides a process for producing a heat-resistant intermetallic compound Ni3Al foil having a room-temperature ductility, which comprises a first step of forming a stating rod of an alloy having a chemical composition containing Ni as a main component and Al by arc-melting, a second step of growing the starting rod in columnar crystal form by unidirectionally solidified rod to form a plate, and a fourth step of cold-rolling the plate at room temperature to form a foil.
The invention second provides the process for producing the heat-resistant intermetallic compound Ni3Al foil, wherein the alloy in the first step contains Al in an amount of at least 12.8% by weight and at most 13.6% by weight and has an Ll2-type ordered structure. The invention third provides the process for producing the eat-resistant intermetallic compound Ni3Al foil, wherein the alloy in the first step contains a third element other than Al. The invention fourth provides the process for producing the heat-resistant intermetallic compound Ni3Al foil, wherein in the first step of rod having a diameter of 50 mm or less is formed as the starting rod. The invention fifth provides the process for producing the heat-resistant intermetallic compound Ni3Al foil, wherein the rate of unidirectional solidification in the second step is 25 mm/h or less. The invention sixth provides the process for producing the heat-resistant intermetallic compound Ni3Al foil, wherein in the third step, the thickness of the plate is 5 mm or less. The invention seventh provides the process for producing the heat-resistant intermetallic compound Ni3Al foil, wherein in the cold-rolling of the plate in the fourth step, annealing is conducted at a temperature of 800xc2x0 C. (or 1,073 K) or more for 20 minutes or more. The invention eighth provides the process for producing the heat-resistant intermetallic compound Ni3Al foil, wherein after the fourth step, the work-hardened, cold-rolled foil is annealed with a degree of vacuum of higher than 10xe2x88x923 Pa at a temperature of 800xc2x0 C. (or 1,073 K) or more for 20 minutes or more, and further cold-rolled to form a foil.
Moreover, the invention ninth provides a heat-resistant intermetallic compound Ni3Al foil having a room-temperature ductility which foil has a chemical composition containing Ni as a main component and Al in an amount of at least 12.8% by weight and at most 13.6% by weight, and has a thickness of 200 microns or less.